The present invention relates to a process and an apparatus for recovering a PFC gas used in semiconductor manufacturing processes such as an etching process.
Certain perfluorinated compounds and fluorohydrocarbon compounds used in semiconductor manufacturing processes, such as etching processes, contribute to the so-called greenhouse effect and remain in a stable state for a long periods, namely several to hundreds of times longer than that of carbon dioxide. Therefore, these remain undecomposed for a long period, and their contribution to warming is thousands to tens of thousands times higher than that of carbon dioxide. Gases which contribute considerably to a greenhouse effect include CF4, C2F 6, C3F8, C2F4, CHF 3, C4F8, NF3, etc., which are generally referred to as PFC (perfluoro compound) gases and pressure has been mounting for control of the emission thereof. At the Kyoto COP3 (Conference on United Nations Framework Convention on Climate Change) held in Kyoto in December 1997, a motion was passed for control of greenhouse effect gas emissions including a 6% reduction in carbon dioxide and other gases to below 1990 levels by 2010, and a 6% reduction of PFC, HFC, SF6 to below 1995 levels by 2010.
PFC gases used in semiconductor manufacturing processes have a comparatively limited influence on global warming, that is, greenhouse effect at present since the rate of emission thereof, is far lower than that of CO2. However, as noted, such PFC gases have a long life in the atmosphere to result in a high global warming factor for 100 years. Thus, the following approaches have been proposed to reduce the emission of PFC gases.
These approaches include optimizing processes by using a circulation cycle or the like to reduce the amount of PFC gases used, recovering PFC gases to recycle them, seeking alternative gases and developing the corresponding processes, or disposal after decomposition and detoxification.
Among them, the method of recovering PFC gases to recycle them uses membrane separation, adsorption separation or deep freeze separation. Membrane separation relies on differences in permeation rates at which gaseous molecules pass through a membrane, and simultaneously accomplishes recovery and concentration by providing an apparatus for recovering and concentrating a PFC gas following use of an apparatus for removing non-PFC gases. Adsorption separation relies on differences in adsorption performance depending on the pressure at which gases are adsorbed to an adsorbent. Deep freeze separation relies on differences in vaporization curves of gases, i.e., differences in the boiling points of gases, as shown in FIG. 1, to selectively separate a target PFC gas by collecting said PFC gas in a trap at a temperature slightly lower than the boiling point of said PFC gas and raising the trap temperature to a temperature slightly higher than the boiling point of said PFC gas.
However, membrane separation involves the following disadvantages.
1) Gas under high pressure needs to be supplied to the membrane, and thus a vacuum cannot be used.
2) membrane recovery can not effect recovery and condensation of a small flow of PFC gas as used in etching or the like. That is, the PFC gas to be concentrated escapes to a permeation side of impure gases. Particularly when the concentration of the PFC gas on the feeding side is high, a larger part of the PFC gas permeates. Thus, the concentration of the PFC gas should be lowered by means of some other gas (N2 or the like) to ensure an adequate flow rate.
3) The concentration of the PFC gas concentrate or the concentration of impurities varies with the concentration or flow rate of the feed PFC gas, O2 or N2. Some means for controlling variation such as a backup tank is required.
4) The gas supplied to the membrane should be pretreated to remove acid gases, or the membrane will degrade. This pretreatment incurs additional costs.
5) These conditions need a powerful and large compressor or a large buffer container or the like, which leads to a large-scale equipment occupying a large installation space.
Deep freeze separation also involves the following disadvantages.
1) Only a PFC gas cannot be condensed and vaporized from a mixture of a plurality of gases, but some other gases of the mixed gas are included. Therefore, a multistage separation and regeneration system is required for a concentration similarly to fractional distillation of gases or liquids, which also requires large-scale equipment.
2) A powerful refrigerator and a large fractionator are required because the multistage fractionator can not be provided in a vacuum piping for the lack of space but must be run under atmospheric conditions on the outlet side of a vacuum pump.
3) Concentration to some extent and a fair amount of gas are needed at the previous stage.
In view of the above, the present invention aims to provide a process and an apparatus for recovering a PFC gas, which can readily bring cooling traps to a cryogenic temperature with a low-capacity refrigerator and can recover a high-purity PFC gas by applying deep freeze separation without the need for using a multistage fractionator.
The invention of claim 1 relates to a process for recovering a PFC gas, comprising freezing and collecting a determined amount of a mixed gas containing the PFC gas discharged from a vacuum processing chamber in a cooling trap, then stopping the operation of said cooling trap and passing the regenerated mixed gas emitted by vaporization of said frozen and collected gas through a non-PFC gas removal system to remove gases other than the PFC gas from said regenerated mixed gas and provide a high-concentration of PFC gas, and recovering said high-concentration PFC gas.
When deep freeze separation is used to freeze and collect a mixed gas containing a PFC gas discharged from a vacuum processing chamber in a cooling trap at or below the temperature which allows the target PFC gas to be trapped as defined above, 100% of the PFC gas can be condensed in principle. On the contrary, membrane separation is not useful for efficiently recovering a PFC gas because the PFC gas to be concentrated escapes during concentration.
The invention of claim 2 relates to an apparatus for recovering a PFC gas from a mixed gas containing the PFC gas discharged from a vacuum processing chamber, comprising a cooling trap connected to the exhaust system of the vacuum processing chamber and adapted to freeze and collect the mixed gas discharged from said vacuum processing chamber, a non-PFC gas removal system for removing gases other than the PFC gas from the regenerated mixed gas emitted by vaporization of said frozen and collected gas after the operation of said cooling trap is stopped, and a recovery means for recovering a high-concentration PFC gas freed of gases other than the PFC gas in said non-PFC gas removal system.
When the cooling trap is connected to the exhaust system of the vacuum processing chamber, i.e. when the cooling trap is provided on the vacuum side as defined above, a small size, low-capacity refrigerator suffices to easily attain a cryogenic temperature. Moreover, a high-concentration PFC gas can be obtained because the non-PFC gas removal system removes gases other than PFC gases (SiF4, CO2, HF, F2 or the like herein referred to as non-PFC gases) from the mixed gas emitted by vaporization of the frozen and collected gas after the operation of the cooling trap is stopped.
The invention of claim 3 relates to the apparatus for recovering a PFC gas according to claim 2, which comprises a first circulation loop for supplying the high-concentration PFC gas discharged from the non-PFC gas removal system to the recovery means and returning a part of the high-concentration PFC gas to the cooling trap, whereby a part of said high-concentration PFC gas is supplied to the cooling trap via said first circulation loop.
When a first circulation loop is provided as defined above to send the high-concentration PFC gas discharged from the non-PFC gas removal system to the cooling trap via the first circulation loop while supplying said high-concentration PFC gas to the recovery means, the regenerated mixed gas in the cooling trap can be rapidly sent over the high-concentration PFC gas to the non-PFC gas removal system.
The invention of claim 4 relates to the apparatus for recovering a PFC gas according to claim 3, which comprises a second circulation loop for returning a part of the high-concentration PFC gas discharged from the non-PFC gas removal system to the inlet side of the non-PFC gas removal system to constantly send the high-concentration gas to said non-PFC gas removal system via said second circulation loop.
When a second circulation loop is provided as defined above to constantly send the high-concentration gas to the non-PFC gas removal system via said second circulation loop, entrapment can be prevented.
The invention of claim 5 relates to the apparatus according to claim 2, which comprises a vacuum pump provided on the inlet side of the non-PFC gas removal system. When a vacuum pump is provided on the inlet side of the non-PFC gas removal system to reduce pressure on the side of the non-PFC gas removal system as defined above, regeneration in the cooling trap can be performed under reduced pressure. Thus, the regeneration temperature of the cooling trap can be lowered to prevent inclusion of water vapor in the regenerated gas and to save the energy required to return the cooling trap to the cooling trap temperature after completion of regeneration.
The invention of claim 6 relates to the apparatus according to claim 5, which comprises a circulation loop for supplying a part of the high-concentration PFC gas discharged from the non-PFC gas removal system as a gas for oil diffusion prevention and dilution to the vacuum pump. The invention of claim 7 relates to the apparatus according to claims 5 or 6, which comprises a first circulation loop for supplying the high-concentration PFC gas discharged from the non-PFC gas removal system to the recovery means and returning a part thereof to the cooling trap, whereby a part of said high-concentration PFC gas is supplied to the cooling trap via said first circulation loop.
The invention of claim 8 relates to the apparatus for recovering a PFC gas according to any one of claims 2 to 7, which comprises two cooling traps wherein a mixed gas containing the PFC gas discharged from the vacuum processing chamber is frozen and collected at one cooling trap while the operation of the other cooling trap is stopped to recover the PFC gas from the regenerated mixed gas emitted by vaporization of the frozen and collected gas.
When two cooling traps are provided and a mixed gas containing a PFC gas discharged from the vacuum processing chamber is frozen and collected at one cooling trap while the cooling operation of the other cooling trap is stopped to recover the PFC gas from the regenerated mixed gas emitted by vaporization of the frozen and collected gas as defined above, continuous processing can be achieved without any interruption in the vacuum processing chamber even when the collecting ability of one cooling trap is lowered and is required to be regenerated, since the other cooling trap serves as a cooling side to freeze and collect the mixed gas discharged from the vacuum processing chamber.
The present invention will now be described by way of examples with reference to the attached drawings. The following examples illustrate a process and an apparatuses for recovering a PFC gas discharged from an etching chamber during a semiconductor manufacturing process. The examples described below and shown in the drawings illustrate the embodiment comprising two cooling traps for parallel operation as defined in claim 8.